An array of armed opposition groups have advanced through the outskirts of Idlib, the provincial capital and home to some 165,000 people, since launching their campaign Tuesday.
Rebels have long controlled most of the surrounding towns and countryside, but President Bashar Assad's forces have maintained their hold on the city.
The Britain-based Syrian Observatory for Human Rights said the rebels have seized at least 17 checkpoints and military outposts from pro-Assad troops. It said at least 11 government fighters and 17 rebels have been killed over the past 24 hours.
A photograph posted today on a Twitter account run by the Nusra Front's Idlib unit showed what it said was the group's black flag being raised over a captured government checkpoint north of the city.
Late yesterday, Syria's state news agency said the army killed "tens of terrorists" on the eastern side of Idlib. The government refers to Assad's opponents as "terrorists.